linux-kernel - Re: Trying to use 'perf probe' to debug perf itself

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Date:	Wed, 25 Feb 2015 10:25:02 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jiri Olsa <jolsa@...hat.com>, David Ahern <dsahern@...il.com>
Subject: Re: Trying to use 'perf probe' to debug perf itself

Em Wed, Feb 25, 2015 at 11:53:16AM +0900, Masami Hiramatsu escreveu:
> (2015/02/25 3:49), Arnaldo Carvalho de Melo wrote:
> > Available variables at thread__get
> >         @<thread__get+0>
> >                 struct thread*  thread
> > [root@...andy ~]#

> > cool, so I thought it would be just a matter of asking to put the probes
> > and get the value of 'thread', then match things, but:

> > [root@...andy ~]# perf probe -v ~/bin/perf thread__put thread
<SNIP>
> > Writing event: p:probe_perf/thread__put /root/bin/perf:0xd03d2 thread=-32(%sp):u64
> > Failed to write event: Invalid argument
> >   Error: Failed to add events. Reason: Invalid argument (Code: -22)
> > [root@...andy ~]#

> > Not possible :-\ 
 
> Hmm, strange. Could you tell me the version of your kernel?

[root@...andy ~]# uname -r
3.10.0-210.el7.x86_64

> It seems that the kernel newer than 3.14 supports uprobes with
> memory dereference (e.g. -32(%sp) )feature.

Right, that must be the case, will test, but then, would it be possible
for the kernel, in such cases, return something line EOPNOTSUP?

I will try to figure out a better error message on the tooling side,
something like:

. Realize we're asking for memory dereference in uprobes
. If it fails with EINVAL, check the kernel version and say something
  like:

Please upgrade your kernel to at least x.y.z to have access to feature
FOO_BAR.

- Arnaldo

> (it was actually introduced by 5baaa59e, and git-describe told
>  it was v3.13-rc4-22-g5baaa59)
> 
> And also, could you try to write the result command to uprobe_event as
> below?
> 
> # echo "p:probe_perf/thread__put /root/bin/perf:0xd03d2 thread=-32(%sp):u64" >> \
>  /sys/kernel/debug/tracing/uprobe_events

Well, I'll try that if it fails after I upgrade to 3.14.

> > please let me know if you need some file, here is the readelf -wi for
> > those two routines:
> 
> This should not be the problem of dwarf-analysis. It seems kernel-side
> (uprobe) problem.

Thanks a lot!

- Arnaldo
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